The present invention is directed generally to a variable diameter power transfer assembly and, more particularly, to the drive transmission for a combine harvester cleaning fan.
Variable diameter power transfer assemblies comprising a drive sheave and a driven sheave for transmitting power between two shafts and of which each sheave is composed of two flanges which are axially adjustable with respect to each other, are well known in the art. Typically, each cooperating pair of flanges forms a V-shaped belt receiving groove, the effective diameter of which can be varied by mechanical, hydraulic or electric means acting on the drive sheave flanges for adjusting the spacing therebetween, whereas compensating axial movement of the driven sheave flanges with respect to each other is controlled by a compression spring forcing said two sheave flanges together. The variable diameter power transfer assembly just described thus is capable of giving an infinite number of velocity ratios between the driving shaft and the driven shaft within a given range of values. An example of such a variable diameter power transfer assembly is disclosed in greater detail in EP-A-0.097.986 to which reference is made.
Due to constructive restrictions however, the mentioned range of velocity ratios is fairly limited and the power transfer assembly preferably is designed to adequately fulfil the common requirements whereby this assembly is unable however to efficiently handle less frequently occurring conditions for which either an excessive speed reduction or a speed increase out of the common would be demanded. Indeed, assuming the variable diameter power transfer assembly is operable to control the rotational speed of a fan incorporated in a combine harvester for assisting in the grain cleaning process, then the variable speed transmission is designed to perform fully satisfactory in crops and crop conditions for which the combine harvester is best suited such as for harvesting so called heavier grains such as wheat and corn. However, with a fan variator designed and set to tackle these heavier conditions, severe difficulties are encountered when harvesting very light grains, such as grass seed for example, which are easily made airborne and run the risk of being blown out of the combine.
One known solution for this recognized problem consists of shielding off the fan air inlets, thereby reducing the airflow through the fan. Unfortunately, the resultant air pattern in the combine cleaning system quite often becomes disrupted, adversely affecting the operation of the combine cleaning mechanism.
Another known solution to this problem has been to replace the variable speed transmission for obtaining a "high" speed range with a second variable speed transmission for obtaining a "low" speed range each time when switching over from harvesting the heavier grains to harvesting the lighter grains. Although functionally acceptable, it will be appreciated by one skilled in the art that this solution of replacing the drive components is expensive in that two variable speed transmissions are needed, and furthermore, this solution is also undesirable because of the time consuming effort involved for dismounting and mounting the respective variable speed transmissions.